Explain the TCP/IP layer model and how it applies to embedded systems.

The TCP/IP model has four layers, each with a distinct responsibility. The Link layer handles physical transmission over a specific medium — Ethernet, Wi-Fi, or cellular. The Internet layer (IP) provides addressing and routing so packets can traverse multiple networks. The Transport layer (TCP or UDP) provides end-to-end communication between applications — TCP adds reliability with retransmissions and flow control, while UDP provides raw datagrams. The Application layer implements the protocol your application actually speaks — HTTP, MQTT, CoAP, DNS, or a custom binary protocol.

In embedded systems, you interact with this model differently depending on your platform. On embedded Linux, the kernel implements the full TCP/IP stack, and your application uses the POSIX sockets API — the same API as a desktop application. You rarely think about the lower layers. On a bare-metal or RTOS system, you typically use a lightweight IP stack like lwIP or Zephyr's native stack. These stacks implement the same layers but with much smaller memory footprints (lwIP can run with as little as 40 KB of RAM) by using memory pools instead of malloc, supporting fewer simultaneous connections, and offering a "raw" API that avoids data copies.

The key embedded-specific considerations at each layer are: the link layer may be constrained (a cellular modem with high latency, a low-power radio with tiny MTU), the IP layer must handle intermittent connectivity gracefully, the transport layer choice (TCP vs UDP) directly impacts RAM usage (each TCP connection requires send/receive buffers), and the application protocol must be bandwidth-efficient (binary formats like CBOR or Protobuf instead of JSON).